THE USE OF SOLAR FURNACES IN MATERIALS RESEARCH

SOLAR ENERGY: INTERNATIONAL PROGRESS

THE USE OF SOLAR 5UINACES IN MATERIALS HESEAIEH D.Suresh Department of Mech. Engg. Indian I n s t i t u t e of S c i e n c e Bangalore-560 0 1 2 , I n d i a .

and

P.K. Itohatgi Director C.S.I«H. Complex Trivandrum, I n d i a .

ABSTRACT S o l a r f u r n a c e s are promising tool·* f o r s t u d y i n g u l t r a h i g h temperature p r o p e r t i e s of m a t e r i a l s . In t h i s paper, we have reviewed the a p p l i c a t i o n s of s o l a r f u r n a c e s i n the f i e l d of m a t e r i a l s r e s e a r c h i n d e t a i l and have b r i e f l y p r o j e c t e d some of t h e i r i n d u s t r i a l a p p l i c a t i o n s . Besearch on c r y s t a l growth, h i g h temperature phase diagrams of oxide s y s t e m s , i g n i t i o n of m a t e r i a l s , p u r i f i c a t i o n and s t a b i l i z a t i o n of r e f r a c t o r i e s i s being conducted u t i l i z i n g t h e v e r y h i g h temperatures a t t a i n a b l e i n s o l a r f u r n a c e s , vdthout any c o n t a m i n a t i o n . S p e c i f i c f u t u r e o p p o r t u n i t i e s f o r r e s e a r c h i n t h e s e areas u s i n g s o l a r furnaces have been i d e n t i f i e d . Some of the f u t u r e r o s p e c t s of s o l a r furnaces c o u l d be the s t u d \ on f r e e z i n g of superheated a l l o y s , z o n e - r e f i n i n g of l a r g e q u a n t i t i e s of m a t e r i a l s , growing new s i n g l e c r y s t a l s f o r semi-conductor and l a s e r a p p l i c a t i o n s , vacuum w e l d i n g and m a t e r i a l s r e s e a r c h i n o u t e r s p a c e . INTH0DÜGTI0N E x t r a c t i o n and p r o c e s s i n g of m a t e r i a l s consumes a ver}' l a r g e f r a c t i o n of w o r l d ' s nonrenewable energy s o u r c e s l i k e c o a l , g a s and o i l . In f a c t , i n f u t u r e , the shortage of t h e s e nonrenewable energy s o u r c e s w i l l r e s t r a i n the production of m e t a l s and m a t e r i a l s . This r e s t r a i n t can be overcome t o a g r e a t e x t e n t by the u t i l i z a t i o n of S o l a r Energy f o r e x t r a c t i o n and p r o c e s s i n g of m a t e r i a l s . In r e c e n t y e a r s i n c r e a s i n g e f f o r t s are b e i n g made t o u t i l i z e s o l a r furnaces f o r r e s e a r c h as w e l l as f o r i n d u s t r i a l a p p l i c a t i o n s . The French CNRS 1000 kW s o l a r furnace i s a landmark of achievement i n t h i s d i r e c t i o n . S o l a r furnaces o f f e r the advantage of a t t a i n i n g v e r y high temperatures more than 3500°C, w i t h o u t any c o n t a m i n a t i o n , magnetic or e l e c t r i c f i e l d i n t e r f e r e n c e and w i t h the p o s s i b i l i t y f o r measuring the temperature v e r y a c c u r a t e l y . Spot m e l t i n g i n a c a v i t y by u s i n g the sample i t s e l f as c r u c i b l e , and w o r k a b i l i t y i n any d e s i r e d atmosphere o r i n vacuum are the major a t t r a c t i o n s of t h e s e f u r n a c e s , i n a d d i t i o n t o , of course the advantage of f r e e f u e l and n e g l i g i b l e maintenance c o s t s . Solar furnaces are v e r y u s e f u l i n the s t u d y of h i g h temperature p r o p e r t i e s of m a t e r i a l s , such a s phase s t u d i e s , c r y s t a l growth, p u r i f i c a t i o n and s t a b i l i z a t i o n of r e f r a c t o r i e s and i n t e s t i n g of m a t e r i a l s a t v e r y h i g h t e m p e r a t u r e s . I n d u s t r i a l a p p l i c a t i o n s l i k e p r o c e s s i n g of m a t e r i a l s , p r e - h e a t i n g s c r a p , upgrading the raw m a t e r i a l s , e x t r a c t i v e m e t a l l u r g y , m e l t i n g and c a s t i n g of m e t a l s i n rural workshops could a l s o be p o s s i b l e . S o l a r welding and b r a z i n g i n vacuum could become a major t e c h n o l o g y of the f u t u r e . Space f a c t o r i e s , i n f u t u r e with a s o l a r furnace on-board could perform m a t e r i a l s r e s e a r c h and produce new m a t e r i a l s f o r use i n e l e c t r o n i c s and o t h e r a p p l i c a t i o n s . At p r e s e n t , Prance and the U.S.S.Ïv are engaged i n e x p l o r i n g l a r g e s c a l e i n d u s t r i a l u t i l i s a t i o n of s o l a r f u r n a c e s , and C h i l e and s e v e r a l c o u n t r i e s are about t o j o i n them i n the n e a r f u t u r e . Japan i s i n v o l v e d e x t e n s i v e l y in b a s i c r e s e a r c h w i t h small s o l a r furnaces and the U.S.A i s o p e r a t i n g m i l i t a r y furnaces f o r m a t e r i a l t e s t i n g and r e s e a r c h . There i s a l s o c o n s i d e r a b l e amount of p r o p e r i e t o r y ' 1443

SOLAR ENERGY: INTERNATIONAL PROGRESS 'work going on in several countries which i s not reported in open literature e CHÏSTAL GHOWTH Demand for solar c e l l s and several other requirements for ultra pure single crystals can be met with, in the near future, by growing crystals in solar furnaces. In conventional methods such as metal flux method and rf-heated floating zone method, i t i s sometimes d i f f i c u l t to grow high quality single c r y s t a l s , because of the high melting point and r e a c t i v i t y of the melt. Solar furnaces are more suitable for these applications because of the a b i l i t y to operate in clean and controlled atmospherfs(Pig.l). Under tiiese conditions, i t will be possible to grow many single crystals which would otherwise decompose or vary in stoichiometry under normal heating conditions. The major results obtained by previous workers in crystal growth usinfr solar furnaces are summarized in Table I · In 1956 Laszlo ( l ) reported the growth of single crystals of Z1O2 in a solar furnace. Later, Färber (2) grew perfect crystals of A^Oßf CaO, MgO, Hf02, TiC and TiB2 by vapour deposition and puddle melt methods, in a solar furnace. The growth of alumina and ruby crystals may be further investigated for use in laser applications. A technique to grow crystals in a desired crystallographic axis could be developed, with suitable temperature control, perhaps using a laser as a stand-by source. Laszlo e t al (3) have grown thoria single crystals from the vapour phase in a 1.5 m solar furnace. Dae to vapour frac t i on at ion, magnesium, the major impurity in the solid thoria, was completely eliminated, while s i l i c o n and iron contents were reduced. Crystals grown by condensation from vapours clearly exhibited the characteristic crystal habit. Crystals as large as 3 mm long and 1 mm in cross section were obtained. The growth rate was 0.5 mm/min at the highest flux available, i e . , 500 cal/sq.cm/sec. Even at the lowest Ðá÷ l e v e l , crystal growth occurred due to the fact that vapour pressure of the solid was s u f f i c i e n t to cause nucleation. Laszlo e t al have also suggested the possible mechanisms for crystal growth out of a continuously receding molten base. The mechanisms of growth of crystals in solar furnaces must be ascertained by systematic studies of varying different parameters in a controlled manner. Sakurai e t al (4) have studied the growth of UO« crystals by a solar furnace, in helium atmosphere. They have suggested a mechanism of this growth, which i s not satisfactory and needs further explanation. The crystals grown had good stoichio­ metry with a pyramidal form and optically f l a t surfaces of size about 0.15 mm. Sakurai and Ishigame (5) have reported the growth of nickel oxide crystals in a 10 m solar furnace 9 for use in their optical and e l e c t r i c a l investigations. Optically f l a t crystals with surface area of about 40 sq.ram were obtained. The r e s i s t i v i t y of the specimen obtained from the melt was found to be reduced drasti­ c a l l y , possibly due to the boiling off of certain impurities and specific l a t t i c e re-arrangements. The stoichiometry of these crystals also showed deviation. The study on the variation of stoichiometry after treatment in a solar furnace will be an interesting area for future research. Crystals may be prepared with desired r e s i s t i v i t i e s from the same starting material by t h i s method, which cannot be e a s i l y done using other methods. Ishigame e t al (6) grew Ni doped CaO single crystals which were a few mm in grain s i z e , in a solar furnace«. Single crystals of CaO were also grown as a reference for optical measurements. Pure CaO crystals were colourless while Ni^ doped CaO crystals were pink in colour and the transmittance was measured in IK, v i s i b l e

1444

SOLAR ENERGY: INTERNATIONAL PROGRESS

Pyrex enclosure

Solar radiation

Inlet for quenching water Inlet for coolant

Gas flow J-Water flow

Evacuation and gas flow

;. A ^ / O S P r i H *

1445

-A^bLf<

(4)

(ä)

(6)

T. S a k u r a i e t a l

T. S a k u r a i e t a l

M. I s h i g a m e e t a l

Y. Nigara et al

4.

5.

6.

7.

(7)

(3)

T.S. Laszlo e t al

3.

1971

1968

1968

1968

1967

1964

(2)

1956

(1)

3.A. Färber

I GI»vTII growifc

USING

Õ ^

, N .i 2" +doped CaO

NiO

UO

ThO

Al 0 , CaO, MgO Ðßâ2? TiC, TiB2

ZrO

Crystal

CIÛTSTAL

T.S. Laszlo

IN Year

DON2 Pusf.

WOÎK

Investigators

0Ã

2.

S I . No.

SUMMAiar

TABLE SOLAIO

crystal

crystals

Single e rys talsi ze afe w mm

Single c r y s t a l s c o l o u r l e s s - f e w ¹¹ i n grain size

Polyc rys t a l 1 i n e , o p t i c a l l y 9 f l a t c r y s t a l s of 40 aim a r e a

C r y s t a l s with pyramidal f o r a of s i z e 0 . 1 5 mm f l a t faces.

Single c r y s t a l s 3 mm X 1 toni X 1 ms

Single crystal

Single

N a t u r e of

PURGES

SOLAR ENERGY: INTERNATIONAL PROGRESS

^ -^

^ι

0.

4.

1.

SI.No.

II

1954

1967

(17)

(18)

(19)

II. Conn

T. No^uclii e t a l

M. Hizuno e t a l

1975

1975

197S

(19)

(20)

(22)

M. Mizuno e t a l

M. Mizuno e t a l

J . P . Coutures e t al

1975

Year

3ef.

Investigators

2°3-Y2°3

(l-x)Al.O, 23 Na 2°3 *

AW-Va

Al203-Ce02

AWVs

A1

Linuidus curve p r e s e n t e d y p t o 4ö?' of a l u m i n a c l e a r jprlass r e s u l t e d and above 50$ of alumina d e v i t r i f i e d

"aV3^

Amorphous p h a s e ::ed a t 900°C.

recrystalli

f2°3 V ^ V P-Ga2°3 p h a s e s found

G

T e n t a t i v e phase diagram given-Two r h a s e r e g i o n between 15 t o 25 mole<3

T e n t a t i v e p h a s e diagram given-New orthorlionibic p h a s e a t 90 mole* CeO

2 ύ

T e n t a t i v e phase diagram p r e s e n t e d—New orthorhonibic phase a t 80 mole$ La 0 .

T e n t a t i v e »hase diagram p r e s en t e d-Pe ale between 40 and GO inoel$ Y o 0 .

/■lass.

lie s u i t s

System s t u d i e d

obtained

SUÏtaiff 0 ? TJ-Ãßú ♦♦'O^K DONE IN P &SS STUDIES OF DIFFERENT SYSTE-ß USINO 'ÔOIAI RJimCES

TAJÌLE

SOLAR ENERGY: INTERNATIONAL PROGRESS

M. Mizuno e t a l

M. Mizuno e t a l

M. Mizuno e t a l

9.

10»

M. Mizuno e t a l

J . P . Coutures e t a l

Investigators

I I (CONTD.)

8.

7.

SI .No.

TABLE

1077

1977

1977

1977

(21)

(24)

(25)

1976

Year

(23)

Bef.

203-Pr2°3

2° 3 - N d 2°3

A1

2°3-Ett2°3

^WVs

A1

A1

System.: s t u d i e d

T e n t a t i v e diagram g i v e n S i n g l e monoclinic phase 2Eu 0 .Al 0 o b s e r v e d Three e u t e c x i c s i d e n t i ­ fied.

T e n t a t i v e dia5^ram g i v e n S i n g l e m o n o c l i n i c phase of 2Sm D . A l 0 o b s e r v e d Three e u t e c t i c s i d e n t i f i e d

T e n t a t i v e phase diagram p r e s e n t e d - PrAlO i d e n t i ­ f i e d i n two forms-Cooling curve of Pr_0 i n a i r showed f rocnage' pheno­ menon

R e v e r s i b l e pliase t r a n s i t i o n of NdA103 a t 1100°C-Two eutectic points identified.

T e n t a t i v e phase diagram given-A new phase NdgOg.ll Al 0 observed.

B e s u l t s obtained

SOLAR ENERGY: INTERNATIONAL PROGRESS

co

T. Noguchi e t a l

T. No:;uchi e t a l

A. Rouanet e t a l

16.

17.

(H)

Ú . Sakurai e t al

15.

1957

(io)

Pol Jjuv/ez e t a l

13.

P . S . Evans

1978

(26)

M. Mi zuno e t a l

12.

14.

1977

(25)

al

M. Miziino e t

11. 203-M203

obtained

T e n t a t i v e phase dia:ra*n içiven. T e n t a t i v e phase .'.ia^ram g i v e n - A n o n a l i e s found a t 3 . 5 , 7 . 0 , 1 0 . 0 aiole;' CaO. Phase d i a c r e , i i n e OLI pi e t e— A new c u b i c l i k e phase a t 1 2 . 5 mole'' MgO-Phase m o d i f i c a t i o n a t 1300°C. Phase dia»;ran fjiven-A new p h a s e - TV.O eu t e c o i e s identified.

Z l o 2 -uo 2

ZrO -CaO

ZrO -ìfeO

ZxO2-La203

19G7

1968

1968

(13)

(14)

(--;

T e n t a t i v e phase diagram given—The m a l e xpans i on ni e a s u r e d-£ o:n p l e t e ph a s e d i a-; r a i ç i v e n .

Tliree s t a b l e and s i n g l e phases i d e n t i f i e d .

T e n t a t i v e d i a ^ r a t Riverì— S i n g l e s t a b l e nionoclinic pha e 2Gd 0 .AlgO o b s e r v e d ïhree eutecxics iclentified.

Ile s u l t s

1960

ZrO - ¹ é 0 ï

Al 0 -Dy 0 Ä 2 3 y2 3

A1

System S t u d i e d

(12)

1975

Year

(COira.) lief.

II

Investigators

SI.No.

'ABLE

SOLAR ENERGY: INTERNATIONAL PROGRESS

A. Houanet e t al

Ô. N o c c h i e t a l

T. Noguchi e t a l

T e No g u a t i

A. Ilouanet

19.

20 â

21.

22.

23.

Investigators

(COIN'JD.)

A. îlouanet e t a l

II

18.

SI»No.

TABLE

1969

1970

1971

(13)

(15)

(16)

Liquidus curve given-Anomalies a t 7 . 5 , 1 5 . 0 and 2 0 . 0 mole£ Y 0 -Biaxial negative crystals a t 15 mole;j ^ñ^ï· Phase diagrams

Zr02-Y2°3

Zr02-Ln203

ûven

T e n t a t i v e phase 'lia^rain g i v e n Freezinj, p o i n t of 00 mole^ SrO, s e n s i t i v e t o s t o i c h i o ìetry-. Two coìnplex compounds o b s e r v e d .

ZrO -SrO

T e n t a t i v e diagram ^ i v e n Anonaly a t 1 7 . 5 nolc# TiO,

Zr02-Ti09

1908

Phase dia^ran 'pLven-À e u t e c t i c p o i n t and a Nev; phase a t hi^ti temperatures on Gd 0 side.

ZrO - G d 0 o 2 2 o

1968

Phase diagram <_>;iven-Eutectic p o i n t a t 75 mole^ Sm 0 Ne·, phase a t h i ^ i temperatures i n Sm_0 s i d e .

R e s u l t s obtained

Zr0

2"" S n i 2 0 3

System Studied

19 08

Year

(13)

—

Ref.

SOLAR ENERGY: INTERNATIONAL PROGRESS

M· Yoshimura e t al

J . P · Coatares

28«

29·

G« Brauer e t a l

M· Yoshimura e t al

30·

31·

F· Sibieada e t a l

F« Sibiendn e t a l

(28)

M. Miz^uno e t a l

27·

(27)

J · Coatares e t a l

26.

1977

(29)

2°3" W 0 3

La203-J*i203

(34)

1977

^ V * ^

Ti02-Ti203

(9)

i i i ) I« 2 0 3 -A0 2

ii)

i ) IHjjOj-sy^

Ce

Th0 2 -lA 2 0 3

i v ) I* 2 0 3 -1«A03 1960

3

I* 2 0 3 -Y 2 0 3

Nd 2 0 3 -Y 2 0

»2VC*208

System Stadied

_ _

1975

1976

1975

1976

1974

1974

1971

Year

(33)

(32)

(31)

J . Couture ir e t a l

25·

(30)

Bef·

G« Benezech e t a l

Investigators

Si.No.

24·

(CCNTD.)

TABLE I I

Rapid quenching using hammer anvil device-Riase diagram presented-Glass formation near the e u t e c t i c .

Phase diagram presented·

Phase diagrams presented

Six s t a b l e phase of cerium tungetates i d e n t i f i e d ·

Tentative phase diagrams given.

Phase diagram given-Several extended solid solutions a t high températures· Liquidus carve determinedTentative phase diagram givenTwo forms of LaY0 o eetablished.

Phase diagram determined by C ofus ion method·

Phase diagram given t

Besaite obtained

SOLAR ENERGY: INTERNATIONAL PROGRESS

SOLAR ENERGY: INTERNATIONAL PROGRESS

3000h

2500h

Cubic s . s .

T.C «P °~

c/i

20001

"δ e o cn o

i_

D

a a E

C.C

1500F

T.C

1000 > C  Monoclinic s.s. 0 ZrO,

10

20

30

_L_

Λ0

JL

50

60

70

80

90

Concentration of Th CU (Mole°/o) FIG. 2. PHASE

DIAGRAM OF Z r 0 2 - T h 0 2

1452

100 ThO,

SOLAR ENERGY: INTERNATIONAL PROGRESS and UV r e g i o n s . One may t r y t o grow s i l i c o n c r y s t a l s doped with s u i t a b l e elements f o r use i n e l e c t r o n i c s i n d u s t r y by a technique u t i l i z i n g s o l a r z o n e - r e f i n i n g . Nigara e t a l ( 7 ) have grown yttrium oxide c r y s t a l s i n a s o l a r furnace and the r e f l e c t i v i t y was measured i n the IR r e g i o n . The c r y s t a l s were o n l y a few mm i n s i z e . The mechanism of growth and the presence of hexagonal s t r u c t u r e may be taken up f o r f u r t h e r r e s e a r c h . Payne ( 8 ) has remarked t h a t s o l a r furnaces are n o t s u i t a b l e f o r drawing s i l i c a f i b r e s because temperature c o n t r o l i n s o l a r furnaces i s n o t e a s y , which i s of paramount importance t o ensure drawing f i b r e s with good d i a m e t e r c o n t r o l . The major q u e s t i o n b e f o r e s o l a r s c i e n t i s t s i s the development of s u i t a b l e i n s t r u m e n t a t i o n t o go with a s o l a r furnace which g i v e s c o n t r o l l e d growth. PHASE DIAGRAM DETERMINATION FOR OXIDE SYSTEMS Study of h i ^ i temperature phase diagrams with s o l a r f u r n a c e s o f f e r s the p o s s i b i l i t y of quenching the fused specimens i n a i r a t c o o l i n g r a t e s of about 1 8 0 0 - 2 3 0 0 ° C / s e c . and the p o s s i b i l i t y f o r s p l a t c o o l i n g . The phase diagram of polycomponent systems can be e s t i m a t e d i n a s o l a r f u r n a c e , u s i n g i t as a s t a t i c as w e l l as a dynamic s y s t e m . I n 1960, Brauer ( 9 ) p u b l i s h e d a phase diagram f o r TiO - T i 2 0 s y s t e m . Other than t h i s , we can broadly c l a s s i f y the systems i n v e s t i g a t e d so f a r u s i n g s o l a r furnaces i n t o , ( a ) z i r c o n i a , ( b ) alumina and ( c ) l a n t h a n i d e based s y s t e m s . Table I I shows the summary of work done by p r e v i o u s i n v e s t i g a t o r s . a ) Z i r c o n i a Systems Pol Duwez e t a l (IO) i n v e s t i g a t e d tiie phase r e l a t i o n s h i p s in the system ZrO - T h 0 9 . The work of Duwez with a s o l a r furnace e s t a b l i s h e d ^ e x i s t e n c e of a c u b i c CaFT type s o l i d s o l u t i o n , f o r t h o r i a c o n c e n t r a t i o n s above 1 7 . 5 mole^. Only a f t e r prolonged h e a t i n g a t 2000°C and a t high temperatures, t h i s s o l i d s o l u t i o n was s t a b l e . At 1350 e C, i t decomposed i n t o t e t r a g o n a l z i r c o n i a and t h o r i a - r i c h phase c o n t a i n i n g a t l e a e t 99 mole$ t h o r i a . During subsequent c o o l i n g , t e t r a g o n a l z i r c o n i a transformed i n t o monoclinic form, through an atomic s h e a r mechanism which c o u l d n o t be suppressed even i f the r a t e of c o o l i n g was as h i g h as 1 5 0 0 ° C / s e c . Since the polymorphic i n v e r s i o n of z i r c o n i a w i l l always be p r e s e n t , t h e s e b o d i e s w i l l be v e r y s e n s i t i v e t o thermal shock. Because o f the inadequate data o n l y a t e n t a t i v e diagram was p r e s e n t e d . L a t e r Sakurai e t a l ( i l ) have g i v e n a complete phase diagram of t h i s system s t u d i e d i n a s o l a r furnace ( F i g . 2 ) . Evans ( 1 2 ) gave a t e n t a t i v e phase diagram f o r ZrO^-UO s y s t e m . Melting i n a s o l a r furnace avoided t u n g s t e n contamination as occurrea i n p r e v i o u s work. From 1350°C, t h e specimens were c o o l e d r e l a t i v e l y s l o w l y and the p o s i t i o n of the e u t e c t o i d ( c i r c a 940°C) could n o t be e s t a b l i s h e d because of the t r a n s f o r m a t i o n from t e t r a g o n a l t o m o n o c l i n i c form i n the f u r n a c e - c o o l e d s p e c i m e n s . E s t a b l i s h i n g the e u t e c t i c p o i n t by a m o d i f i e d method may be undertaken t o d e v e l o p a complete phase diagram. Noguchi and h i s coworkers ( l 3 ) systems u s i n g a s o l a r f u r n a c e . quenched specimen showed ZrTiO m e l t i n g compounds. An anomaly a t 1 7 . 5 mole$ T i 0 9 , i n the ZrO

have s t u d i e d the ZrO -TiO , ZrO -SrO, and ZrO -CaO I n the ZrOg-TiO system s t u d i e d by them, the ,monoclinic ZrO and t e t r a g o n a l TiO as i n c o n g r u e n t l y i n the l i ^ u i d u s curve of the system nad been found - r i c h r e g i o n . Z i r c o n i a had shown two exothermic

1453

SOLAR ENERGY: INTERNATIONAL PROGRESS peaks i n the c o o l i n g curve due t o s t r u c t u r e m o d i f i c a t i o n from cubic t o t e t r a g o n a l and i t was found t h a t quenching could not f r e e z e the h i g h temperature ZrO m o d i f i c a t i o n . S t a b i l i z a t i o n s t u d i e s may f u r t h e r be extended which could r e s u l t i n a d i f f e r e n t c l a s s of r e f r a c t o r i e s . In the ZrO -CaO system, anomalies i n the l i q u i d u s curve were n o t i c e d f o r 3 . 5 , 7 . 0 and 1 0 . 0 molefo CaO a t 2 6 0 3 ° , 2622° and 2578°C r e s p e c t i v e l y . The m e l t was analysed t o c o n t a i n only rhombic form. But the r e l a t i o n between cubic ZrO and cubic z i r c o n i a s o l i d s o l u t i o n , i n and above the temperature range of 2200° t o 2800°C need f u r t h e r i n v e s t i g a t i o n . The c r y s t a l s t r u c t u r e of z i r c o n i a above 2500°C, a t which a new s t a b l e z i r c o n i a s o l i d s o l u t i o n may e x i s t , i s y e t t o be s t u d i e d . In the c a s e of ZrO^-SrO system, the f r e e z i n g p o i n t s of v a r i o u s c o m p o s i t i o n s were examined, and i t was found t h a t the f r e e z i n g p o i n t of t h e composition 60 inole^ SrO was v e r y s e n s i t i v t o v a r i a t i o n i n s t o i c h i o m e t r y . A t e n t a t i v e phase diagram was g i v e n . The e x i s t e n c e of complex compounds S r Zr . 0 (n = 2 , 3 , 4 ) and SrZr0 o was confirmed. n

n—l

oYi—c.

o

I n the ZrOp-MgO system s t u d i e d by Noguchi e t a l ( l 4 ) the composition of 1 2 . 5 raole^ MgO quenched from the m e l t showed a new cubic l i k e phase, which e a s i l y decomposed i n t o monoclinic and t e t r a g o n a l forms by r e h e a t i n g a t 1300°C. To p r e s e n t a complete phase diagram, the c r y s t a l m o d i f i c a t i o n above 2300°C has t o be i n v e s t i g a t e d . The presence of compounds l i k e Mg o Zr o 0 ft and MgZro0 was n o t confirmed s a t i s f a c t o r i l y in t h i s study. Noguchi and h i s coworkers ( 1 5 ) have a l s o s t u d i e d the ZrO -Y p 0 system. The c o m p o s i t i o n of 1 5 . 0 molefc Y 9 0« quenched from the melt showed a new cubic phase. Y t t r i a showed an exothermic peak i n the c o o l i n g curve a t about 2283°C i n the s o l i d s t a t e , due t o the cubic hexagonal t r a n s i t i o n . Kouanet ( 1 6 ) gave the phase diagrams f o r ZrO - L a ^ ZrO -Px- 0g ZrO^Nd O3, Z ^ - T b ^ , ZrO - D y ^ , ZrO -Çï,,Ï Zr02E r 2 0 3 , ZrOg-Y^Ojj and Z™2 2 3 sysïems· b) Alumina Systems Conn ( 1 7 ) s t u d i e d the system A1.0 -SiO and gave the l i q u i d u s c u r v e , which agreed with the r e s u l t s obtained by p r e v i o u s workers who used a d i f f e r e n t method. Noguchi and Mizuno Î 1 8 ) p r e s e n t e d the phase diagram f o r A l 9 0 -Y 0 s y s t e m s . Mizuno e t al ( l 9 ) determined the phase diagram f o r the Al^O.-La 6 system a t higji temperatures i n argon atmosphere and the presence of a new ortnorhombic phase was e s t a b l i s h e d a t a c o m p o s i t i o n of 8 0 . 0 molefo La0Q ( F i g . 3 ) . A h i g h temperature phase diagram was p r e s e n t e d f o r the system A1"0 -CeO , e s t a b l i s h i n g a new orthorhombic phase a t a composition of 9 0 . 0 mole$ Ceo â They ( 2 0 ) have g i v e n a t e n t a t i v e phase diagram f o r Al-O -Ga_0 system ( F i g . 4 ) , a f t e r e s t i m a t i n g the l i q u i d u s c u r v e . The s o l i d s o l u t i o n Between «*C- ^ ï 0 « * and B -Ga 0 c o n t a i n e d a two phase r e g i o n a t the c o m p o s i t i o n range of 15 and 25 mole$ Gra-0«. Quenched & - G a - 0 . gave the n e e d l e l i k e c r y s t a l s . Same authors ( 2 I ) gave a t e n t a t i v e diagram a t h i g h temperatures, f o r the A l p 0 - P r 0^ system ( F i g . 5 ) . They have e s t a b l i s h e d rhombohedral and cubic forms f o r tue compound PrAlO a t 950°C. Cooling curve of Pr Q 0 i n a i r showed the rochage phenomenon, f o r which the reason has t o be e s t a b l i s h e d . The presence of PrO as s u g g e s t e d by them has y e t t o be v e r i f i e d . Coutures ( 2 2 ) published the r e s u l t s on the system ( l - x ) A 1 2°3"" x N d 2°3· ^ a n 0 r P h o u s phase was obtained by rapid quenching v/ith a hammer-anvil d e v i c e v ^ i g · 6 ) in a s o l a r f u r n a c e . Their study showed t h a t the amorphous phase was indeed a g l a s s ,

1454

SOLAR ENERGY: INTERNATIONAL PROGRESS

2400h Liquid

Liq 

La 2 0 3 (x)~ 2200 Liq*La 2 0 3 (H)Z- — .Liq i í LaË 02OiA)ST ú

2OOOP

Liq* 'LaAlOa

Liq* \Liq*R LaAl0 3 1840

1800h

R* La203(A)

R*LaAl03

1780

1890 1Τ10

1600h LaAl0 3 * ί α 2 0 3 ( Α )

/3~Al 2 0 3 *LaAl0 3

0

10

20

30

AO

50

60

70

80

Al203

90 100 Ld203

Concentration of L a 2 0 3 (Mole°A>) F I G . 3 . PHASE

DIAGRAM

1455

OF Al 2 0 3 -La 2 0 3 INARGON.

SOLAR ENERGY: INTERNATIONAL PROGRESS

2400h

Liquid

2200h

14001

L

0 10 Al203

20

30

40

_i

50

1

60

L

70

80

90 100

Ga203

Concentration of G a 2 0 3 (Mole 7o) FIG.4. PHASE DIAGRAM OF A l 2 0 3 - G a 2 0 3 IN AIR.

1456

SOLAR ENERGY: INTERNATIONAL PROGRESS

;ocF

>00l·

)oob

*ool·

1819

1800

Md-s < I

0

PrAl03 *Pr2 0 3

/3Al203*PrAl03

10

20

30

40

50

60

70

80

90

Al203

100

Pr203 Concentration of

FIG. 5. PHASE

Pr 2 0 3

(Mole°/o)

DIAGRAM OF A l 2 0 3 - P r 2 0 3 IN AIR

1457

SOLAR ENERGY: INTERNATIONAL PROGRESS

Solar radiator

To pump

FIG.6. HAMMER-ANVIL DEVICE USED IN A SOLAR FURNACE.

1458

SOLAR ENERGY: INTERNATIONAL PROGRESS

£i»lAJ

1 Liquid

2200h

21 55

,2071 2000

2303,

/

/

Λ \ L^

// Li ^*

Liq\pAΞ203

Al203\

/



\

/ Liq^ \

/

/ N O AIO3 Nd AIO 3 \

/ Nd 2 0 3 _^/ Ο85Τ

/

1800 J_79_5 V

Al203 !

Liq

/ V 1720

p

1600 _A1203 NdAl03tNd203

P-Al 2 0 3 + NdAl0 3

u

0

10

1

20

1

30

1

40

50

1

60

1

70

1

80

Al203

1

90 100 Nd203

Concentration of N d 2 0 3 (Mole 7o) FIG.7. PHASE

DIAGRAM OF A i 2 0 3 - N d 2 0 3 IN AIR.

1459

SOLAR ENERGY: INTERNATIONAL PROGRESS which r e e r y s t a l l i z e d a t about 900°C. In another study using s p l a t cooling device a new ?>hase was obtained in the neodymium rich p a r t of Al 0 -Nd 90 system. In 1977 Mizuno e t a l (23) proposed a t e n t a t i v e phase diagram of the same system (Fig.7) from the cooling curve measurements. Two e u t e c t i c points vere observed with 23 and 80 molef» Nd 0 a t 1720°C and 1850°C r e s p e c t i v e l y . They observed the presence of Nd 0 .1L\1 0 , in a fused specimen, coexisting with NdAlO . At 1100°C,NdAlO was seen to undergo reversible phase t r a n s i t i o n from rhombohedral to cubic. Later on these authors (24) published a t e n t a t i v e phase diagram for Al 0 -Sm 0 system a t high temperatures. A s i n g l e , monoclinic phase of 2Smo0 .Al 0 with freezing point a t 1977°C was observed. Three e u t e c t i c points a t Ï775°, 1910°, 1880°C respectively a t 24,61 and 80 mole^ Sm 0 were observed. At about 750°C, there was a reversible phase t r a n s i t i o n between orthorhombic and rhombohedral forms. They (25) recently gave t e n t a t i v e phase diagrams for the systems Al p 0 -Eu 0 and Al o 0 -Gd 0 . Each of the quenched snecimen from the melt showed single îonoclinic phases of 2Eu 0 . Al 0 and 2Gd 0 . ^IQ^O r e s p e c t i v e l y . These compounds showed no phase t r a n s i t i o n in repeated heading and cooling c y c l e s . Al o 0 -Eu 0 system shoved three eutectico a t 1710°, 190)°, and 1860°C with 24, 62 and ?7 raole^ En 0 . In the Al 0 -Gd 0 system, three e u t e e t i c s - t 1720°, 1920° and 1930°C vitli 25, 05 and 73 rnole^ & Ï ï were observed. Recently Io. luno e t al (26) gave the r e s u l t s of t h e i r study on Al 0 -13y 0 system. Quenched specimens from the melt showed three stable and ^ V single pSa&es, namely S D y ^ . S A l ^ , DyA103 and ^ y ^ . c

) Lanthanide Systerns

Coutures e t al (27) have sho n t h a t the La 0 -Y o 0 system foimed several extended solid solutions a t high temperatures, the phases of which were very d i f f e r e n t from those observed with products cooled down to room temperature. A phase diagram was presented with no experimental data on liquidus curves. Mizuno e t a l (28) have given the liquidus curve for t h i s system with the i n v e s t i g a t i o n on c r y s t a l modification of quenched specimens from the melt with a aolar furnace. A t e n t a t i v e phasr* ^1.1 ram a t high temperature was presented ( F i g . 8 ) . The presence of orthorhombic and monoclinic forms of LaY0q was e s t a b l i s h e d . Coutures (29) has obtained the phase diagrams for the r a r e - e a r t h sesquioxide systems a t high temperatures. For a b e t t e r knowledge of these diagrams, s p l a t cooling process has been adopted. The systems were anal3rsed using ?I-ray diffractometry and theimogravinetrie d i f f e r e n t i a l scanning calorimetry. The systems studied were I n A n \ o 0 (Ln = La, Nd, Sm, Gd, Dy, Er, Yb ; A = Al, C r ) , Ln 0 -Ln 0 , Ln o 0 -A0 \ X = Zr, Th) and LnQ0 -LnAO ( A = Cr, W). Benezech e t al(30) have given*" a high temperature phase diagram'-on Yb O.-Cr 0 system melted in a i r in a s o l a r furnace ( F i g . 9) · In 1974, Coutures e t a l (31) have given the phase diagrams for Nd 0 -Y 0 and ^ ï ^ ï - ^ ï ^ ï systems studied by cofusion method in a i r in a 2 kW s o l a r furnace. 2 <3 2 ς Sibieude and Foex (o2) have studied the various phases and phase t r a n s i t i o n s in the systems ThO -Ln p 0 ( where Ln = La, Nd, Sm, Gd, To, By, Er, Y) from ambient temperature upto the melting p o i n t . Tentative phase diagrams have also been presented. As an example, the high temperature phase diagram of the system ThO -Y-0 i s c i t e d in Fig. 10. <-> 2 o

1460

SOLAR ENERGY: INTERNATIONAL PROGRESS I n 1976 Yoshimura e t a l ( 3 3 ) s t u d i e d t h e system Ce 0 -WO i n a 2 kW s o l a r f u r n a c e u s i n g c o f u s i o n m e t h o d · Molten b a l l s were c o o l e d a t a r a t e of 10 K / s e c by s p l a t c o o l i n g method u s i n g a m e c h a n i c a l hammer ( F i g · l l ) · High t e m p e r a t u r e X - r a y methods were employed f o r p h a s e s t u d i e s · S i x s t a b l e p h a s e s of c e r i u m t u n g s t a t e s were i d e n t i f i e d . They c o u l d n o t o b t a i n any p h a s e o t h e r t h a n t h e m o n o c l i n i c , a t room t e m p e r a t u r e even by a s p l a t c o o l i n g method f o r Ce p 0 .3W0 . F u r t h e r s t u d i e s f o r s i n g l e c r y s t a l s a t h i g h t e m p e r a t u r e s c o u l d be d o n e · They h a v e a l s o s t u d i e d t h e 0 | u b - l i q u i d u s p h a s e r e l a t i o n i n t h e t e r n a r y s y s t e m CeOg-Ce-O -WO . A r e c e n t worlijCYoshimura e t a l ( 3 4 ) h a s been on t h e system La.O -V0 m e l t e d i n a s o l a r f u r n a c e and r a p i d l y q u e n c h e d · Usin& a h a m m e r - a n v i l d e v i c e , a c o o l i n g r a t e of 10 ° C / e e c was a c h i e v e d « A p h a s e d i a g r a m h a s been p r e s e n t e d · Near tile e u t e c t i c c o m p o s i t i o n , g l a s s f o r m a t i o n was o b s e r v e d · The c o l o u r of g l a s s p h a s e ranged from c o l o u r l e s s t o p a l e g r e e n . F u r t h e r s t u d y on the v i s c o s i t y of t h e m e l t s c o u l d be d o n e · The low t e m p e r a t u r e s t r u c t u r e s of La 0 #2W0„ and La 0 eW0 c o u l d be d e t e r m i n e d · A s t u d y on v a r i o u s f a c t o r s a f f e c t i n g t h e g l a s s f o r m a t i o n such a s t h e m e c h a n i c a l s t r e s s b y c o n t a c t w i t h m e t a l w a l l s c o u l d be c o n d u c t e d i n f u t u r e · IGNITION STUDIES F o r i g n i t i o n of any m a t e r i a l , s a y a p r o p e l l e n t , u n l e s s e n e r g y e x c e e d i n g t h e t h r e s h o l d v a l u e i s s u p p l i e d , i g n i t i o n w i l l n o t o c c u r · C o r r e l a t i o n s t u d y between t i m e and e n e r g y w i l l h e l p t o p r e d i c t f o r example t h e l i k e l y flame s p r e a d b y i r r a d i a t i o n due t o n u c l e a r e x p l o s i o n s · M i l i t a r y f u r n a c e s l i k e t h e F r e n c h LCA s o l a r f u r n a c e and t h e U . S . Army WSSF a r e used t o t e s t m a t e r i a l s i n a s i m u l a t e d n u c l e a r e x p l o s i o n e n v i r o n m e n t · H i s t o r i c a l l y , s o l a r i g n i t i o n s t u d i e s s t a r t e d i n ihe Archimedian a g e · But a r e a l s c i e n t i f i c b a s i s f o r u s i n g t h e s o l a r f u r n a c e i n i g n i t i o n t e s t s , was l a i d by Newton· Lomonoeov ( 3 5 ) c o n s t r u c t e d a c a t a d i o p t r i c igniting instrument· I n 1954, Gardon ( 3 6 ) r e p o r t e d t h e r e s u l t of i g n i t i o n s t u d i e s on m a t e r i a l s i n t h e MIT s o l a r f u r n a c e · R e l a t i v e l y l a r g e a r e a had t o be i r r a d i a t e d t o t r e a t t h e h e a t t r a n s f e r problem a s one d i m e n s i o n a l · When wood was s u b j e c t e d t o i g n i t i o n t e s t s , t h e smoke and d e c o m p o s i t i o n p r o d u c t s formed a p r o t e c t i v e l a y e r and e v e n t u a l l y t h e e f f e c t i v e r a d i a t i o n was r e d u c e d · W e l l s ( 3 7 ) h a s a l s o e x p e r i e n c e d a s i m i l a r d i f f i c u l t y d u r i n g h i s s t u d i e s on a d i f f e r e n t s y s t e m · When t h e sample e n c l o s e d i n g l a s s m e l t e d , t h e v a p o u r c l o u d e d t h e e n c l o s u r e and t u r n e d o f f t h e h e a t « P r o b a b l y t h i s phenomenon c o u l d be u s e d i n vacuum c o a t i n g of m a t e r i a l s o v e r a s u b s t r a t e f o r example r e f r a c t o r i e s on p l a s t i c · MATERIAL TESTING AND STUDIES ON THERMOPHYSICAL PROPERTIES AT HIGH TEMPERATURES S o l a r f u r n a c e s a r e i m p o r t a n t i n s t u d y i n g t h e b e h a v i o u r of m a t e r i a l s s u b j e c t t o v e r y s e v e r e t h e r m i c i m p u l s e s o r c y c l e s i n any a m b i e n t c o n d i t i o n · The development of a s u i t a b l e m a t e r i a l which w i l l w i t h s t a n d v e r y h i g h t e m p e r a t u r e s and t h e r m a l s h o c k s h a s g r e a t m i l i t a r y and s c i e n t i f i c i m p o r t a n c e , and f i n d s immediate application in several f i e l d s · U . S . N a t i o n a l Bureau of S t a n d a r d s ( 3 8 ) s e t up a 5 f t ( l . 5 m) s o l a r f u r n a c e i n t h e 1 9 5 0 ' s f o r s t u d i e s on t e m p e r a t u r e - r e s i s t a n t m a t e r i a l s t o w i t h s t a n d t h e e x t r e m e c o n d i t i o n s found i n atomic r e a c t o r s , a i r ­ c r a f t e n g i n e s and g u i d e d m i s s i l e s . The l a b o r a t o i r e C e n t r a l de l'Armement h a s b u i l t on t h e P y r e n e e s O r i e n t a l s , a complex c o m p r i s i n g a main s o l a r f u r n a c e of

1461

SOLAR ENERGY: INTERNATIONAL PROGRESS

Liquid 2Θ00Κ

^Liq «-Xss

0 Yr 5 u0 , 2

10

20

30

40

50

60

70

80

3

90

100 La203

Concentration of La 2 0 3 ( Mole 7o ) FIG.8. PHASE DIAGRAM OF Y2 0 3 - La 2 0 3 IN AIR

1462

SOLAR ENERGY: INTERNATIONAL PROGRESS

2500| 9/rri.

Cr

—g 2°3'-YbCr 03

g Yb 2 0 3 2 -Yb Cr 0 3

Cr ο ( . 2 2 0 0 ί , 1 0 β α ( 2 3 3 o S : ) < 2 1 1 0 i 1 ° *C>

Cr

2100h

2°3 ( s ) * E i ( l ) YbCr0 3 (s)*E,(l)

YbCr0 3 (s)*E 2 (0 Yb 2 0 3 (s)*E 2 (l

200Q190d- Cr 2 0 3 (s)*YbCr 0 3 (s) 180Q 0 ΞO Cr203

/

20

30

«5

YbCr0 3 (s)*Yb 2 0 3 (s) 50

60

Concentration of Yb 2 0 3

FIG.9. PHASE

70

80

90 100 Yb 2 0 3

(Mok%>

DIAGRAM OF Yb 2 0 3 - C r 2 0 3 IN AIR.

1463

SOLAR ENERGY: INTERNATIONAL PROGRESS

3200

2800u

2^00l·

|

2000I

C(Th0 2 ) / C ( T h 0 2 ) * H

l H.C (Y2O3)

CL

I

1600h 12001 0 Th 0 2

C(Th0 2 )*C(Y 2 0 3 )

20

J

40

.!_. J

60

W2d

1_ 1

80

L_

100 Y203

Concentration of Y 2 0 3 (Mole7o) FIG.10. PHASE DIAGRAM OF Th 0 2 - Y 2 0 3 .

1464

SOLAR ENERGY: INTERNATIONAL PROGRESS

Mechanical hammer

• A Heat rays

χ Water :ircula1ion

|

^'_

!_"_

/

i

^ C o p p e r support

^•Sample

FIG.11. MECHANICAL HAMMER FOR SPLAT COOLING I N A SOLAR FURNACE.

1465

SOLAR ENERGY: INTERNATIONAL PROGRESS 42 lui power and a secondary furnace of 400 W. These LCA furnaces have been designed to d e l i v e r b r u t a l thermic impulses onto m a t e r i a l s ( 3 9 ) , The speed of a p p l i c a t i o n and amount of energy provided to the t a r g e t are the important parameters i n such a pyrogenic s t u d y . U . S . Army s o l a r furnace a t the White Sands M i s s i l e ftan^e i s a l s o b e i n g used t o t e s t m a t e r i a l s i n a simulated r a d i a n t thermal en e rgy e rivi ronmen t . Otts e t a l ( i o ) have reported some r e s u l t s of m a t e r i a l t e s t i n g i n the 5 mW S o l a r Thermal T e s t F a c i l i t y . I t has been found t h a t c o n c r e t e w i l l withstand 500 to 1000 suns and w h i t e , e x p o s e d - a g r e g a t e c o n c r e t e over 1000 suns f o r upto 2 m i n u t e s , w i t h o u t s e v e r e m e l t i n g . Commercial alumina f i r e b r i c k s can w i t h s t a n d upto 2500 suns f o r upto 2 minutes w h i l e a l u m i n a - s i l i c a based ceramic i n s u l a t i n g m a t e r i a l s can withstand 1500 suns f o r over one m i n u t e . Z i r c o n i a based ceramic i n s u l a t i n g m a t e r i a l s can withstand 2500 suns f o r over 4 m i n u t e s . A s i m i l a r t e s t - r e s u l t was announced e a r l i e r by Sakurai e t a l ( 4 l ) . A r e f r a c t o r y b r i c k (SK35) melted even in f e e b l e s u n l i g h t w i t h i n a minute, i n t h e i r 70 kW s o l a r f u r n a c e . Coutures ( 2 9 ) has reported the r e s u l t s of a study i n a 2 kW s o l a r furnace on thermophysical p r o p e r t i e s of r e f r a c t o r y m a t e r i a l s i n v o l v i n g r a r e - e a r t h s e s q u i o x i d e s , which could be used a^s e l e c t r o d e s f o r the M3D p r o c e s s . The MRD m a t e r i a l s must have good mechanical and e l e c t r i c a l p r o p e r t i e s such as pure e l e c t r o n i c conduction witii good e m i s s i o n . These ÚßÖ e l e c t r o d e m a t e r i a l s must a l s o have good r e f r a c t a i r i t y and must r e s i s t v a p o r i z a t i o n and c o r r o s i o n a t h i g h t e m p e r a t u r e s . The s o l i d s o l u t i o n s of z i r c o n i a - y t t r i u m s e s q u i o x i d e s e r i e s are a promising c l a s s of m a t e r i a l s f o r MS) a p p l i c a t i o n s , because of t h e i r h i g h r e f r a c t o r y n a t u r e . Addition of rare e a r t h o x i d e s produces v a r y i n g d e g r e e s of e l e c t r o n i c c o n d u c t i v i t y f o r t h e s e s o l i d s o l u t i o n s . Noguchi e t a l ( l 3 ) have s t u d i e d the s t o i c h i o m e t r i c compound SrZrO f o r use as MHD i n s u l a t o r and conductor m a t e r i a l . A study on thermophysical p r o p e r t i e s of t e r n a r y Y 0 -ZrO - I n 0 cubic s o l i d s o l u t i o n s on the z i r c o n i a r i c h s i d e i s b e i n g conducted a t O d e i f l o in 1 c o l l a b o r a t i o n w i t h the USSR I n s t i t u t e of High Temperature. ΟΊΙΊΕΕ APPLICATIONS OF SOLAR FUIiNACES Bigg e t a l ( 4 2 ) s o l a r furnace. evaporation and h e a t i n g meteors

s t u d i e d the i c e - n u c l e a t i n g p r o p e r t i e s of n e t e o r i t i c m a t e r i a l in a Their attempt was t o d u p l i c a t e the small p a r t i c l e s formed by r e c o n d e n s a t i o n during the f l i g h t of metoors i n the a t i o s p h e r e , by a t a low p r e s s u r e i n a 12 f t (o.G ra) s o l a r f u r n a c e .

Trombe e t a l produced s i n t e r e d r e f r a c t o r i e s such as zirconium o x i d e , 3 calcium z i r c o n a t e and alumina i n s o l a r f u r n a c e s . S i n t e r i n g of f e r r i t e compacts i n a s o l a r furnace has been done a t Keio U n i v e r s i t y . Ugh temperature mechanical, thermal, o p t i c a l and e l e c t r i c a l p r o p e r t i e s of v a r i o u s m a t e r i a l s have been s t u d i e d w i t h s o l a r furnaces ( 4 3 , 4 4 , 4 5 , 4 6 ; . The p r e p a r a t i o n of the h o t - j u n c t i o n bead of thermocouple was s u g g e s t e d by L a s z l o ( l i · S o l a r welding i s an important a p p l i c a t i o n of s o l a r f u r n a c e s , as given by Uglov ( 4 7 ) · He has given a c a l c u l a t i o n of the power required f o r welding a d e v i c e . A g e n e r a l thermal balance equation i s d e r i v e d on the assumption of a l i n e a r and slow-moving s o u r c e , f u l l p e n e t r a t i o n of the weld, c i r c u l a r weld pool w i t h diameter equ^l to e l d width which i s i d e n t i c a l on both f a c e s , and a mean pool temperature of 1 . 1

1466

SOLAR ENERGY: ..ERNATIONAL PROGRESS times the melting temperature. Frantsevich e t al (48) have very recently reported the résulte of their studies on technological processes l i k e welding in the 861-6 solar furnace of 2 m aperture under cosmic simulated conditions. The radiant flow ranged from 150-1900 W/sqcm. Various alloys i n the form of 0*25 - 2 mm thick f l a t plates (or cylinders) with weld j o i n t s were used. The e f f e c t s of radiant flow e f f i c i e n c y , process speed, and j o i n t type on the s i z e and shape of the weld pool were studied· Solar vacuum welding and brazing could be a very powerful technique in the near future· Solar furnaces operating in space factories can be used for obtaining new materials with special characteristics (49)· Very recently some work has been und er talien a t the General Electric Company, by Cline and Anthony ( 5 0 ) · They have u t i l i z e d a 5.5 m solar furnace to process s i l i c o n waters by thermomigration, which requires an intense, uniform source of heat with a high colour-tempe rature· But they feel that the energy savings afforded by the use of free sunlight i s incidental relative to the value added to the s i l i c o n wafer by the the rmomi grati on process·

i

Solar furnaces have also been used in photo-chemistry· Guiljemonat ( 5 l ) used a 18.4 m diameter solar furnace for studying nitrogen fixation process· Marcus e t al (52) reported an exploratory e f f o r t at Stanford Research I n s t i t u t e to study the photochemical reactions in liquids in open or closed v e s s e l s , irradiated by keeping them at the focus of a solar concentrator. But, for other chemical processes the intermittant operation of solar devices might be a handicap. There may however be some intermittent chemical processes which could yet be studied with solar furnaces. Solar Chemical engineering i s a novel f i e l d which has been i n i t i a t e d by the French Scientists» Flamant e t al (53) have reported a preliminary design and investigation of solar chemical reactors. In a fluidised bed reactor of 2 kW solar furnace, s i l i c a sand fluidized particles were heated and some applications such as decomposition of CaC0_, and thermochemical cycles were realized as f e a s i b l e . Other industrial applications of solar furnaces, for example in extractive metallurgy, mineralogy, refractory industries, and others w i l l be presented elsewhere (54) â SUMMâK? AND CONCLUSIONS In this paper we have summarized the possible applications of solar furnaces in various f i e l d s of materials research and industry, and tried to identify the opportunities for further research. Crystal growth in a solar furnace i s s t i l l a developing f i e l d and there are^several important research problems yet to be solved., Ishigame's work on Ni doped CaO c r y s t a l s has revealed that there i s a p o s s i b i l i t y of growing do-»ed crystals with desired r e s i s t i v i t y . The change in r e s i s t i v i t y , the deviation from stoichiometry of the resulting crystals and the mechanism of such a crystal growth can be studied in future, by varying the parameters such as surrounding atmosphere, irradiation, quenching rates and the i n i t i a l tlieimal history of the specimen. There i s ample scope for detailed study on single crystal growth of semiconductor materials like s i l i c o n doped with suitable materials which v i l i find immediate applications in e l e c t r o n i c s . Coupling solar zone-refining techniques, this method could be useful for obtaining crystals- of pure alumina and zirconia whic'i may be used in e l e c t r i c a l and optical studies. New uses of the crystals of t i t a n i a grown in a solar furnace may be thought of. With innovations in temperature control and instrumentation solar furnaces can become an important

1467

SOLAR ENERGY: INTERNATIONAL PROGRESS tool for crystal grovth. Extensive work on high temperature phase diagram determination has been done using solar furnaces. However most of the diagrams are tentative and need to be finalized through further research. These diagrams are incomplete either because of insufficient data or due to unavoidable polymorphic inversions taking place and lack of refinement in experimental techniques. In the case of zirconia based systems, especially ZrO -ThO . ZrO -U0 and ZrO -TiO the phase transformations can not be suppressed ana these systems are sensitive to thermal shock. Stabilization of such systems over a wide range of temperature will be an interesting research on a new class of refractory materials. Noguchi's work on ZrO -SrO systea has shown that the freezing points of the composition 60 mole$ SrO i s very sensitive to variation in stoichiometry. This may be a basic t e s t for estimating the various chemical compositions. Coutures has coupled splat cooling with a hammer-anvil device in a solar furnace to study the phase diagrams of a series of lanthanide systems. An amorphous glass which recrystallizes at 900eC has been obtained in the case of ( i - x ) A l 9 0 - x Nd 0 system. The uses of such new amorphous glasses may be an area for future research. Stable, single and monoclinic phases have been identified in Al 0 -Gd 0 and Al 0 -Eu 0 systems. Bare-earth sesquioxides are found to be very useful as high Temperature materials usable inside atomic reactors as duct walls and as conductor materials in M3ÏÏ) applications and tliese may be made in solar furnaces. Purification of materials in a solar furnace i s one of the most promising fields for future research. Walter's work on the relative v o l a t i l i t y of seven oxides from a melt has shown that s i l i c o n v o l a t i l i t y i s very different from that of aluminium which i s the l e a s t v o l a t i l e component. The dependence of alkali v o l a t i l i t y on oxygen pressure has also been found. This study can be extended for instance, in purifying Al-Si a l l o y s . These alloys could be superheated either in vacuum or in any desired atmosphere and splat cooled in a solar furnace. This superheating would result in the boiling off the impurities from the liquid metal. Because of a rapid cooling i t may be possible to produce ultrafine grain materials, amorphous materials and even materials with different crystal structures. The vacuum evaporation can also e f f e c t i v e l y be used for vacuum deposition of a variety of refractories on metals and p l a s t i c s , Graphitizing of coconut s h e l l s and fihàjfe and pinjipple and jute fibres using solar furnaces could be other areas of possible research. Brauer found that the melting of TiO resulted in a non-s toi chiome trie composition, depending upon temperature and oxygen pressure. Study on t i t a n i a which i s abundantly available in India may be undertaken. Solar melting of metal-metal, metal-ceramic and ceramic-ceramic eutectic systems and directional s o l i d i f i c a t i o n of reinforced eutectica formed in a solar furnace are the other promising fields« There are several opportunities for industrial uses of solar furnaces. Flamant1 s recent work in extractive metallurgy, such as thermal decomposition of phosphate ores in a solar furnace and beneficiation of the by-products, caälead to large-ecale industrial u t i l i z a t i o n of solar furnaces for mineral beneficiation. Heating clay, ilmenite, raonazite and other minerals in a desired atmosphere to ultra high temperatures in a solar furnace, direct extraction of metals may be achieved. Solar smelting units (as installed in the USSR) could become widely used in future. Solar vacuum welding is another good prospect. This could be a powerful technique in electronics industry, and for welding reactive metals like titanium for applications in vacuum sealing. Foundry operations, sintering of refractory materials, metals

1468

SOLAR ENERGY: INTERNATIONAL PROGRESS

and compounds, pre-heating scrap and charje.materials, h e a t - t r e a t m e n t , melting and c a s t i n g , and dehydration of ores can be done by e s t a b l i s h i n g small scale s o l a r furnaces throughout the country. As Sakurai has envisaged, i t will be possible in near future to operate a s o l a r furnace in outer space. Metals would melt in high vacuum, v i t h o u t any n e c e s s i t y for a c o n t a i n e r or holder, and be c r y s t a l l i s e d free from any s t r a i n which might be introduced by the differences of specific g r a v i t y between l i q u i d and solid s t a t e s . Thus s o l a r furnaces stationed a t the noon or space f a c t o r i e s could produce new m a t e r i a l s h i t h e r t o unknown. ACIÜTOWLEDGEMENTS

V:e wish to express our sincere thanks to Mr. M. Jose for the preparation of i l l u s t r a t i o n s and to Tata Energy Hesearch I n s t i t u t e , I n d i a , for financial support. We are grateful to Dr. C.S. Menon, Dr. P.M. Mohandas and Mr. G. Natrajan for t h e i r comments on the manuscript.

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SOLAR ENERGY: INTERNATIONAL PROGRESS

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2.

Farber, E . A . , ' C r y s t a l s of high-temperature m a t e r i a l s produced i n the s o l a r f u r n a c e ' , S o l a r Energy, V o l . 8, No. 1 , 1964, pp. 38-42 (Ceram. Abs. Aug. 1904, 2 3 0 b ) .

3.

L a s z l o , T . S . e t a l , 'Tharia s i n g l e c r y s t a l s grown by vapour d e p o s i t i o n i n a s o l a r f u r n a c e ' * J . Phy. Chem. S o l i d s , V o l . 2 8 , 1967, pp. 3 1 3 - 3 1 6 .

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Sakurai, T. e t a l , 'Uranium d i o x i d e c r y s t a l s grown by a s o l a r f u r n a c e ' , J . Crys. G r . , V o l . 2 , 1968, pp. 3 2 6 - 3 2 7 .

5.

Sakurai, T. e t a l , 'Growth of n i c k e l oxide c r y s t a l s by s o l a r furnace J . Crys. G r . , V o l . 2 , 1963, pp. 2 8 4 - 2 8 6 .

6.

I s h i g a n e , M. e t a l , ' E l e c t r o n i c a b s o r p t i o n spectrum of Ni J . Phy. S o c . Japan, V o l . 2 5 , N o . 6 , 1968, pp. 1629-1632.

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Nigara, Y. e t a l , ' I n f r a r e d p r o p e r t i e s of Yttrium o x i d e ' , J . Phy. Soc.Japan, Vol. 30, No. 2 , 1971, pp. 4 5 3 - 4 5 8 .

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2+

fusion1,

i n CaO '

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15.

Noguchi, T . , 'Tlie l i q u i d u s curve of the Zr0 o -Y o 0 system as measured by a s o l a r f u r n a c e ' , B u l l . Chem. S o c . Japan, Vol"; 4 3 , 1970, p p . 2614-2616.

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Tl^ouanet, A . , ' C o n t r i b u t i o n a l ' é t u d e des systemjfe zirconeoxydes des l a n t h a n i d e s an v o i s i n a ;e de l a f u s i o n ' , Jtev. I n t . Ilautes Temper, e t l ì e f r a c t . j V o l . 8, 1971, pp. 1 6 1 - 1 8 0 .

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SOLAR ENERGY: INTERNATIONAL PROGRESS Noguchi, T. e t a l , 'Phase diagram of the system Al 0 - Y J ) ' , Kogyo Kagaku Zasshi, Vol. 70, 1967,jp. 834-837. d ό 2 ό Mizuno, M. e t a l , 'High temperature phase s t u d i e s on the system A l 2 0 3 - I a i p 0 w i t h a s o l a r f u r n a c e ' , Paper p r e s e n t e d a t t h e ISEC, Los A n g e l e s , 137o, p p . i - 6 » Mizuno, M. e t a l , "The l i q u i d a s curve i n the system Al O.-Ga 0 a s measured w i t h a s o l a r f u r n a c e ' , Beports of the GIHIN, V o l . XXIVf No. 8 , 1 9 7 5 , pp.184-187* Mizuno, M. e t a l , 'Phase diagram of the system Al 0 - P r 0 q a t h i g h t e m p e r a t u r e , ' Y o g y o - X y o k a i - s h i , V o l . 8 5 , No. 1 , 1977, p p . 2 4 - 2 9 ? Coutures, J . P . e t a l . 'Obtention e t e f t u d e s des v e r r e s r e f r a c t a i r e s a base d'alumine e t de sesquioxyde de neodyme', Mat. R e s . B u l l . , V o l . 1 0 , No. 6 , 1975, p p . 5 3 9 - 5 4 6 . Mizuno, M. e t a l , 'Phase diagram of the system A l p 0 -Nd 0 2 Yogyo-Kyokai-Shi, V o l . 8 5 , No. 2 , 1977, pp. 90-951

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ίΉ3Γ*-*

Mizuno, M. e t a l , 'Hiase diagrams of the system A l p 0 -Eu 0 and Al O.-Gd 0 a t 2 ó h i g h t e m p e r a t u r e s , ' Yogyp-Xyokai-Shi, V o l . 8 5 , N o / i l , 1977,1^^3-548. Mizuno, M. e t a l , 'High temperature s t u d i e s on the system A l p 0 - I n 0 w i t h a • s o l a r f u r n a c e , ' Paper p r e s e n t e d theISSC'77, New D e l h i , J a n . 1978, p p . 1 6 8 2 - 1 6 8 3 . Coutures, J . e t a l , 'Etude a haute temperature dm diagramme d ' é q u i l i b r e du système forme par l e sesquioxyde de lanthane avec l e susequoxyde d ' y t t r i u m ' , J . S o l . S t a t e . Chem., V o l . 1 1 , No. 4 , 1974, pp. 2 9 4 - 3 0 0 . Mizuno, M. e t a l , 'Phase diagram of the system La 9 0 -Y 0 e t h i g h t e m p e r a t u r e s ' , Yogyo-Kyokai-Shi, V o l . 8 4 , No. 7 , 1976, p p . 342-347? à ó Coutures, J . P . , ' S o l a r f u r n a c e s experiments f o r thermophysical p r o p e r t i e s s t u d i e s of r a r e - e a r t h o x i d e s MÎÎD m a t e r i a l s , ' Paper p r e s e n t e d a t t h e 13th Rare-Earth Research Conference, Oct. 1 9 7 7 . Benezech, G. e t a l # ' D i s p o t i f s d* a n a l y s e thermique pour l ' é t u d e des t r a n s f o r m a t ­ i o n s c r i s t a l l i n e s e t des changements de phase a haute t e m p e r a t u r e ' , C o l l o q u e s I n t e r n a t i o n a u x (CNRS) No. 2 0 5 , O d e i l l o , S e p . 1 9 7 1 , p p . 5 7 - 6 9 . Coutures, J . e t a l , 'Etude a haute temperature des systems formes p a r ^ e s e s q u i o x y d e de neodyme avec l e s s e s q u i o x y d e s d ' y t t r i u m e t d ' y t t e r b i u m ' , Mat. R e s . B u l l . , V o l . 9 , No. 1 2 , 1974, p p . 1 6 0 3 - 1 6 1 2 . S i b i e u d e , F . e t a l , 'Phases e t t r a n s i t i o n s de phases a haute temperature o b s e r v e s dan l e s systems ThO - I n 0 (Lu = Lanthanide e t y t t r i u m ) ' , J . Nue. M a t . , V o l . 5 6 , 1975, p p . 2 2 9 - 2 3 8 . ö Yoshimura, M. e t a l , ' I d e n t i f i c a t i o n of b i n a r y compounds i n the system C e 9 0 _ 2 W0 3 ' T J . S o 4 . S t . Chem. ; Vol. 16, No. 3£, 1976, p p . 2 1 9 - 2 3 2 .

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Gardon, R., ' Segnen t e d-m i r r o r s o l a r furnace f o r h i g h i n t e n s i t y thermal r a d i a t i o n s t u d i e s , ' Rev. S c i . I n s t r . , V o l . 2 5 , No. 5 , 1954, pp. 4 5 9 - 4 6 3 .

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38.

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39.

Jaymes, M. ' S o l a r furnaces of Laboratoire Central de l'Aimement', Rev. I n t . Hautes Temp. R e f r a c t # / V o l . lap No. 4 , 1975, pp. 3 0 1 - 3 0 6 .

40.

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